Project title: Understanding the roles of miR-1s in regulating heart development Project Summary: Congenital heart defects (CHD) are the most common birth defects in humans, resulting in significant mortality and morbidity. The overall goal of this project is to investigate the role of microRNA (miRNA) mediated gene regulation, in heart development. miRNAs are recently identified small RNAs that bind through a poorly understood mechanism to the 34-UTRs of their target mRNAs, inducing mRNA cleavage. The PI previously described two muscle-specific miRNAs, miR-1-1 and miR-1-2. Both miR-1s are highly enriched in the cardiac conduction system (CCS) during cardiogenesis and in adulthood. Deletion of miR-1-2 causes partial embryonic lethality. miR-1-2 null survivors develop cardiac conduction defects but are without obvious cardiac structural abnormalities. Overexpression of miR-1-2 at postnatal stages causes AV block of various degree and sinus bradycardia. The PI hypothesizes that the proper function of each component of the CCS requires a precise dosage of miR-1. The PI proposes three specific aims to test this hypothesis: 1) To determine the role of miR-1s in regulating specification and proliferation of the CCS; 2) To determine how different dosages of miR-1 regulate proper function of each component of the CCS; 3) To identify the molecular mechanism by which miR-1s regulate cardiac development. The proposed studies may uncover new fundamental mechanisms of heart disease. Insights from the studies will facilitate steps towards understanding of functions of numerous miRNAs in human diseases.